Probabilistic assessments of clinical care are essential for quality care. Yet, machine learning, which supports this care process has been limited to categorical results. To maximize its usefulness, it is important to find novel approaches that calibrate the ML output with a likelihood scale.
Connolly et al BMC Bioinformatics (2017) 18:361 DOI 10.1186/s12859-017-1736-3 METHODOLOGY ARTICLE Open Access A nonparametric Bayesian method of translating machine learning scores to probabilities in clinical decision support Brian Connolly1, K Bretonnel Cohen2, Daniel Santel1, Ulya Bayram1 and John Pestian1* Abstract Background: Probabilistic assessments of clinical care are essential for quality care Yet, machine learning, which supports this care process has been limited to categorical results To maximize its usefulness, it is important to find novel approaches that calibrate the ML output with a likelihood scale Current state-of-the-art calibration methods are generally accurate and applicable to many ML models, but improved granularity and accuracy of such methods would increase the information available for clinical decision making This novel non-parametric Bayesian approach is demonstrated on a variety of data sets, including simulated classifier outputs, biomedical data sets from the University of California, Irvine (UCI) Machine Learning Repository, and a clinical data set built to determine suicide risk from the language of emergency department patients Results: The method is first demonstrated on support-vector machine (SVM) models, which generally produce well-behaved, well understood scores The method produces calibrations that are comparable to the state-of-the-art Bayesian Binning in Quantiles (BBQ) method when the SVM models are able to effectively separate cases and controls However, as the SVM models’ ability to discriminate classes decreases, our approach yields more granular and dynamic calibrated probabilities comparing to the BBQ method Improvements in granularity and range are even more dramatic when the discrimination between the classes is artificially degraded by replacing the SVM model with an ad hoc k-means classifier Conclusions: The method allows both clinicians and patients to have a more nuanced view of the output of an ML model, allowing better decision making The method is demonstrated on simulated data, various biomedical data sets and a clinical data set, to which diverse ML methods are applied Trivially extending the method to (non-ML) clinical scores is also discussed Keywords: Statistics, Nonparametric, Bayesian, Calibration, Machine learning Background Clinical decision support systems can be defined as any software designed to directly aid in clinical decision making in which characteristics of individual patients are matched to a computerized knowledge base for the purpose of generating patient-specific assessments or recommendations that are then presented to clinicians for consideration [1, 2] They are important in the practice of medicine because they can improve practitioner performance [1, 3–5], * Correspondence: john.pestian@cchmc.org Department of Biomedical Informatics, Cincinnati Children’s Hospital Medical Center, 3333 Burnet Ave., MLC 7024, Cincinnati, OH 45229-3039, USA Full list of author information is available at the end of the article clinical management [6, 7], drug dosing and medication error rates [8–10], and preventive care [1, 11–16] Machine learning (ML) gives computers the ability to learn from, and make predictions on the data without being explicitly programmed regarding the characteristics of that data [17] It should not be surprising, then, that ML pervades clinical decision support, for two reasons First, clinical decision support systems are structured such that patients are represented as features which can be used to map them to categories [18] Second, healthcare data are complex - they can be distributed, structured, unstructured, incomplete, and not always generalizable © The Author(s) 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated Connolly et al BMC Bioinformatics (2017) 18:361 Although logistic regression is widely used in biomedicine and it is highly recommended over ML approaches, ML algorithms have been used in many modern clinical decision support systems, ranging from predicting the incidence of psychological distress in Alzheimer’s Disease [19] to post-cardiac-arrest neuroprognostication [20] A Google Scholar search of “machine learning biomedical” renders over 385,000 results However, there is a problem when ML algorithms are used for clinical decision support The output of a ML model is usually a real number that is thresholded to produce a binary output This outcome appears to come from a “black box”—a system module whose functioning is opaque Yet, caregivers and patients prefer probabilistic statements [21–27] But this “black box” approach runs counter to the goal of improving the decisionmaking power of physicians by providing more – not less – information to make better decisions [28] In other words, “this patient has a 51% chance of developing heart disease” is more informative than a binary output of: “a ML algorithm has indicated that this patient belongs to a group of patients that develops heart disease.” The effect of expressing clinical results probabilistically has been studied for decades As early as 1977, Shapiro [29] introduced a method for assessing the predictive skills of physicians versus the results of “computerized procedures” that had been designed to provide probabilistic predictions of various clinical outcomes Hopkins [30] suggested optimal plain-language descriptions of probabilities in a clinical setting Grimes and Schulz [31] found that combining an accurate clinical diagnosis with likelihood ratios from ancillary tests improved diagnostic accuracy in a synergistic manner Along these lines, Wells et al [32] and Kanis et al [33] provided specific examples of how probabilistic assessments of proximal deep vein thrombosis and bone fracture risk, respectively, could improve clinical outcomes Presenting results in probabilistic terms is as important to patients as it is to clinicians Doctors using the decision-making probabilistic process will give information to patients about risks and benefits, often in numerical terms [34, 35] Trevena et al [36] found that patients have a more accurate understanding of risk if probabilistic information is presented as numbers rather than words, even though some may prefer receiving words The goal of this article is then to ensure that both patient and clinician can gain as much information as possible, and in the most straightforward way possible, from the output of an arbitrary ML algorithm by effectively converting ML-generated outputs to probabilities The assumption here is that the clinician is uninterested in a simple cut-off, but wants to gain an intuitive sense to what degree the ML classifier “believes” that a datum Page of 12 belongs to one class or another But for those who desire a threshold, the calibration is all the more important, since the rational choice of one class over the other is determined by whether the class probability is greater or less than 0.5 There are three common calibration methods used to calibrate ML outputs to probabilities today: Platt Scaling [37], Isotonic Regression [38], and Quantile Binning, which are discussed in turn [39] Platt’s method fits a logistic regression (LR) model to the ML scores from a training set, thereby providing an equation that directly transforms an ML-based classifier score to a probability Although the LR model is not always appropriate and is prone to overfitting for small training sets, it can provide good calibration in certain circumstances (e.g., when Support Vector Machines are used as classifiers) In an attempt to improve upon Platt’s method, the isotonic regression (IR) approach releases the linearity assumptions in the LR model, fitting a piece-wise constant non-decreasing function to the sorted ML scores in the training set Although this calibration can yield good results, the isotonicity assumption is not always valid In fact, Niculescu-Mizil and Caruana [40] demonstrated, using multiple classifiers and data samples of varying size, that both the Platt and IR methods can produce biased probability predictions Quantile Binning, on the other hand, mitigates the assumptions in the Platt and IR approaches by sorting the ML scores from a training set, and partitioning them into subsets (bins) of equal size A new ML score can be simply transformed to a probability by locating its corresponding bin, and then calculating the fraction of positive outcomes in this bin from the training set [39] While less restrictive than the other approaches, the drawbacks of this method include the fact that the number of bins must be set a priori, and that small training sets can corrupt the calibration The Bayesian Binning in Quantiles (BBQ) method mitigates these limitations by effectively averaging over many binning schemes, which leads to a better overall calibration [41] While it is difficult to argue with the overall accuracy and generalizability of the BBQ method, the present work will demonstrate that the granularity and dynamic range of calibrated probabilities, and in some cases the calibration accuracies, can be substantially improved by applying a novel non-parametric Bayesian approach As with the previous methods, this approach requires a training set But rather than using it to build a mapping between ML outputs and probabilities, the distributions of ML output from the positive and negative classes are directly compared to the ML output in question, rendering a probability that the ML output is derived from the one distribution versus the other Connolly et al BMC Bioinformatics (2017) 18:361 Since the ML output is compared to the ML outputs of the two classes, a non-parametric approach is required, as there is no obvious binning strategy Although there are many non-parametric Bayesian methods for comparing two-samples [42–45], non-parametric Bayesian methods for specifically quantifying the probability of distribution pairings (i.e., comparing the similarity of distribution A and B versus the similarity of A to C) are rare Capitalizing on its power and simplicity, the Bayesian non-parametric two-sample comparison approach in Holmes et al [46], is modified for this purpose The improved calibration then arises from the non-parametric approach that effectively allows for an infinite number of binning schemes, and from naturally including statistical uncertainties due to finite training samples The methodology is tested on a variety of data sets that have been classified using two different ML techniques It will be found that the method provides probability estimates with a high granularity within a broad range of calibrated probabilities This is important for many clinical applications For example, in risk assessment studies routinely performed by institutional review boards, government agencies, and medical organizations, it is crucial to be able to compute probabilities that are typically